1. Field of the Invention
The present invention generally relates to a method for optimizing the tripping velocity of a drill string. The present invention is particularly useful for maximizing the efficiency of a drilling operation while maintaining the hydraulic pressure of the drilling fluid in the borehole within an acceptable range. More particularly, the present invention maximizes the velocity at which a drill string may be run into or pulled out of a borehole while not producing surge or swab pressures which are outside a desired safety range.
2. Description of the Background
Rotary drilling operations employ a drill bit for cutting into earth formations to produce a borehole. The drill bit is affixed to the end of a drill string, comprising a plurality of coupled together pipe joints, each about 30 feet in length. Additional pipe joints are added to the drill string as drilling progresses.
During the drilling operation, it is often necessary to remove the drill string in order to change the drill bit. It may also be necessary to remove the drill string for other reasons, such as to perform desired logging operations. After the bit is changed or after such logging operations are completed, the drill string must be reconstructed and returned to the borehole. The process of pulling out and running in the drill string, known as "tripping", is well known in the industry. Tripping is a tolerated and necessary evil which must be completed in as short a time as possible; there is no progress being made in completing the drilling operation during the tripping procedure, resulting in the loss of valuable rig time.
Although pipe joints may be added or removed individually, in order to save time and maximize the efficiency of the drilling rig, it is a general practice to add or remove pipe sections during tripping operations in interconnected lengths of two or three pipe joint sections. These sections are known, respectively, as "doubles" or "trebles" and are often collectively referred to as "stands." Such interconnected stands are normally stored vertically in pipe stands or racks on or near the drilling rig.
In a typical tripping operation, a double or treble pipe stand will be pulled out of or run into the borehole through the rotary table of the drilling rig by use of a vertically movable traveling block in the rig derrick. The double or treble stand is disconnected or connected just above the floor of the drilling rig while the drill string remaining in the borehole is firmly grasped by slips in the rotary table. This operation is continued sequentially until the entire drill string has been pulled out of the borehole or until the drill string has been run into the borehole so that the drilling bit has been lowered into contact with the bottom of the borehole. The operation of pulling out or running in the drill string thus occurs in a plurality of pulls or runs wherein the pipe is accelerated from a stationery position to a pulling or running speed and then decelerated to another stationery position, generally in 60-90 foot intervals. The running or pulling speed is generally constant over substantially the entire length of each of these stepped intervals.
A drilling fluid, commonly known as drilling mud, is continuously circulated down the drill string and up the annulus formed by the drill string and the borehole in order to maintain sufficient pressure in the borehole to impede the entrance of formation fluids into the borehole in order to prevent a well "blowout". It is desirable to maintain the hydrostatic pressure of the static drilling fluid column in the well in balance with the bottomhole formation pressure. However, while a pipe string is being pulled out or run into a borehole, a variety of hydraulic effects create pressure differentials in the drilling fluid relevant to the hydrostatic borehole pressure.
Surging or "surge pressure" involves an increase in the total hydraulic pressure in the borehole to a pressure greater than the normal hydrostatic pressure for the static drilling fluid column in the well. This condition often occurs when the drill string is being run into the borehole at an excessive speed. Excessive surge pressure may result in damage to the surrounding formation, even producing undesirable fractures in the formation. Such fractures may result in the loss of drilling fluid, sticking of the drill pipe and other undesirable results.
Another hydraulic effect, swabbing, may also be produced. Swabbing or "swab pressure" involves a reduction in the total hydraulic pressure in the borehole to a pressure less than the normal pressure for the hydrostatic pressure of the static drilling fluid column in the well. This condition often occurs when the drill string is being pulled out of the borehole at an excessive speed. Such excessive reduction in the hydraulic pressure may result in formation fluids entering the borehole in a condition known as "kick". Additional reduction of the hydraulic pressure may result in an uncontrolled kick or blowout. Further, in soft formations, excessive swabbing may result in collapse of the borehole walls.
Accordingly, it is necessary to minimize the swab and surge pressures associated with pulling out the drill string or with running in the drill string during the tripping operation. However, because the tripping operation is non-productive, it is also desirable to maximize the speed with which these operations are performed so that the down time of the drilling rig is minimized and the efficiency of the rig is maximized. Commonly, a tripping schedule or listing providing suggested optimal tripping velocities per stand is available to the driller. The driller then attempts to pull out or run in the stands of the pipe string at the suggested uniform velocity by noting the total time required for moving each stand of pipe. However, these suggested tripping velocities do not provide maximized velocities per stand in order to minimize the total tripping time while maintaining the drilling fluid pressure within acceptable limits. Accurate and maximized tripping velocities are actually affected by many factors. The maximum, safe velocity may be effected by the amount of moving pipe in a hole, by the depth to which the pipe extends and by the characteristics and pressures of the formations through which the borehole has progressed.
Others have attempted to solve this problem by merely establishing maximum tripping velocities which should not be exceeded and by monitoring the actual tripping velocity to sound a warning signal if the established maximum velocity is exceeded. See the velocity measurement and warning system disclosed by Smith in U.S. Pat. Nos. 3,866,468 and 3,942,594. Others have attacked this problem by determining the volume, and hence the weight or mass, of drilling fluid required to fill the borehole as pipe is pulled out or of drilling fluid displaced from the borehole as pipe is run in. See the comparison system of Leonard disclosed in U.S. Pat. Nos. 3,646,808 and 3,729,986.
However, these efforts have not resulted in a solution to the long felt but unresolved need for a method for optimizing the tripping velocity of a drill string by maximizing the tripping velocity while maintaining the hydraulic pressure of the drilling fluid within an acceptable range.